Landscape Mulch With Fungus

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Organic mulches, like shredded wood, chipped wood, bark, pine needles, compost and shredded leaves are used around trees, shrubs and smaller plants and structures because they can suppress weeds, conserve soil moisture, improve a site's appearance and create a protective buffer around plants. It may cause some concern when a fungal growth appears in a mulched area, although most fungi on mulch are considered, at worst, a nuisance.

About Fungi in Mulch

Organic mulches naturally decompose over time, often gradually improving soil fertility and structure. Different types and sizes of mulch material decay at different rates. For example, bark chunks from softwood trees will resist decay much longer than shredded leaves. As the mulch breaks down, it can support the growth of various decay fungi and other organisms. These fungi do not harm plants and often appear suddenly after heavy rains or watering. Improperly stored or laid mulches are particularly prone to fungal invasions.

Problematic Fungi

The artillery fungus, also known as the shotgun fungus, can cause problems when it grows next to cars, buildings and other landscape items. These fungi produce fruiting structures that look like tiny cups and hold a spore mass that looks like a small black egg. The cuplike portion of the fungus absorbs water and builds up pressure until it shoots the spore mass up in the air towards the light or light-colored objects. The spore masses stick to surfaces and resemble tar spots. The spore masses are difficult to remove and leave stains.

Slime molds, not technically fungi, are bright yellow or orange slimy masses that can grow more than 1 foot across and are often considered unpleasant enough to need treatment. The fruiting body of the stinkhorn fungus has an unpleasant odor, not unlike dung, intended to attract flies that will spread its spores. Some mushrooms growing in mulch may be poisonous, potentially creating problems where small children play around the mulched area.

Preventing Fungi

Apply mulch in a layer no more than about 3 inches thick and make sure the mulch is loose, not compacted or clumpy. Shredded wood and bark or similar mulches that absorb moisture readily are more likely to host decay fungi than materials that do not hold much water. Thoroughly soak any mulch that was bagged or stored in piles immediately after you apply the mulch. This will allow beneficial bacteria to colonize the mulch and compete with nuisance fungi.

Addressing Fungi

You shouldn't use chemicals to try to control nuisance fungi. Remove fruiting bodies as soon as they appear to prevent them reproducing. Regularly rake the mulch to encourage drying and to break up any layers in the mulch that are potentially interfering with water movement. This will help minimize the growth of fungi. You can either remove slime mold and compost or dispose of it or use a hose to spray a forceful stream of water at the mold to break it up. If the mulch fungi continue to be problematic even when you regularly remove fruiting bodies, consider removing and replacing the fungus-hosting mulch.